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TEMPERATURE INVERSION.—A layer in which temperature increases with altitude.

TERTIARY CIRCULATION.—The gen-erally small, localized atmospheric circulations. They are represented by such phenomena as the local winds, thunderstorms, and tornadoes.

THERMAL.—(1) Pertaining to temperature or heat. (2) A relatively-small-scale rising current of air produced when the atmosphere is heated enough locally by Earth’s surface to produce ab-solute instability in its lower layers. The use of this term is usually reserved to denote those cur-rents either too small and/or too dry to produce convective clouds; thus, thermals are a common source of low-level clear-air turbulence.

THERMAL GRADIENT.—The rate of varia-tion of temperature either horizontally or vertically.

THERMAL HIGH.—An area of high pressure resulting from the cooling of air by a cold underlying surface, and remaining relatively sta-tionary over the cold surface. 

THERMAL LOW.—An area of low at-mospheric pressure resulting from high temperatures caused by intense surface heating. They are stationary with a generally weak and dif-fuse cyclonic circulation. They are non-frontal.

THERMAL WIND.—The mean wind-shear vector in geostrophic balance with the mean temperature gradient of a layer bounded by two isobaric surfaces.

THERMOSPHERE.—The atmospheric shell extending from the top of the mesosphere to outer space. It is a region of more or less steadily increasing temperature with height, starting at 70 or 80 km.

THICKNESS.—In synoptic meteorology, the vertical depth, measured in geometric or geopotential units, of a layer in the atmosphere bounded by surfaces of two different values of the same physical quantity, usually constant-pressure surfaces.

THICKNESS CHART.—A type of synoptic chart showing the thickness of a certain physically defined layer in the atmosphere. It almost always refers to an isobaric thickness chart, that is, a chart of vertical distance between two constant-pressure surfaces. It consists of a pattern of thickness lines either drawn directly to data plot-ted on the chart or, more commonly, drawn by the single graphical process of differential analysis.

THICKNESS LINE.—A line drawn through all geographic points at which the thickness of a given atmospheric layer is the same; an isopleth of thickness.

TORNADO.—A violently rotating column of air, pendant from a cumulonimbus cloud, and nearly always observable as a "funnel cloud" or tuba.

TRADE-WIND CUMULUS.—The charac-teristic cumulus cloud in average, undisturbed, weather conditions over the trade-wind belts.

TRADE-WIND INVERSION.—A character-istic temperature inversion usually present in the the trade-wind streams over the eastern portions of the tropical oceans.

TRADE WINDS.—The wind system, occupy-ing most of the tropics, that blows from the subtropical highs toward the equatorial trough.

TRIPLE POINT.—Term commonly used to denote the apex of an occlusion.

TROPICAL AIR.—A type of air whose characteristics are developed over low latitudes. Maritime tropical air (mT) is produced over the tropical and subtropical seas, while continental tropical air is produced over subtropical arid regions. 

TROPICAL CYCLONE.—The general term for a cyclone that originates over the tropical oceans. By international agreement, tropical cyclones are classified according to their intensity (the strength of their surface winds).

TROPICAL DEPRESSION.—A tropical cyclone having a slight surface circulation (at least one closed isobar) and surface winds less than 34 knots.

TROPICAL DISTURBANCE.—An area of disturbed weather over the tropical oceans that often develops into a tropical cyclone.

TROPICAL EASTERLIES.—A term applied to the trade winds when they are shallow and ex-hibit a strong vertical shear. With this structure, at about 5,000 feet the easterlies give way to the upper westerlies, which are sufficiently strong and deep to govern the course of cloudiness and weather. They occupy the poleward margin of the tropics in summer and can cover most of the tropical belt in winter.

TROPICAL STORM.—A tropical cyclone whose surface winds have attained speeds between 34 and 63 knots.

TROPOPAUSE.—The boundary between the troposphere and stratosphere, usually charac-terized by an abrupt change of lapse rate.

TROPOSPHERE.—That portion of Earth’s atmosphere extending from the surface to the tropopause; that is, the lowest 10 to 20 km of the atmosphere.

TROUGH.—An elongated area of low at-mospheric pressure; the opposite of a ridge.

TRUE NORTH.—The direction from any point on Earth’s surface toward the geographic North Pole; the northerly direction along any pro-jection of Earth’s axis upon Earth’s surface, for example, along a longitude line. Except for much of navigational practice (which uses magnetic north), true north is the universal 0° (or 360°, mapping reference.

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